Thermoluminescence dosimetry (TLD) is a technique for measuring radiation doses, using a dosimeter. The dosimeter contains thermoluminescence (TL) elements made of a TL phosphor. When exposed to radiation, a TL element gets activated by the radiation energy. Thermoluminescence is a release of this absorbed radiation energy in the form of light which occurs when the TL element is heated. The amount of light energy discharged from the TL element corresponds to a dose of the radiation received by it. The amount of light energy discharged from the TL element is calculated by integrating the intensity of the thermoluminescence over a time it is observed.
The TLD has been developed to monitor environments in radiation fields. One particular application of the TLD is to monitor doses of radiation sustained by personnel who work in nuclear power plants. Each person working in a nuclear power plant is required to carry a dosimeter called a "TLD badge." The badge contains a TLD plate with one to four TL elements on it. Each person must wear the badge so that the badge will be exposed to the same dose of radiation as the person wearing the badge. Periodically, the badges are processed through a TLD reader to obtain an exposure record for each person being monitored. In the TLD reader, the TL elements on each TLD plate are heated by a heating device, such as an infrared lamp, and thermoluminescence from the elements is detected by a photomultiplier tube and processing circuitry. The detected thermoluminescence is then analyzed by an associated computer. Currently, processing of the TL badges is fully automated, and the TLD readers are capable of successively processing a large number of badges without human intervention. An example of such an automatic TLD reader is a Panasonic TLD Reader UD-710 or a Panasonic TLD reader UD-7900M.
The intensity of thermoluminescence from the TL elements is a function of heating temperature and time. To obtain accurate reading of radiation doses, the TL elements must be heated to a particular temperature for a particular period of time. The optimum heating temperature and time are selected through experiments. In the TLD reader mentioned above, the heating device is operated with programmed parameters so as to heat all the TL elements uniformly to the same optimum temperature for the same optimum time. However, the heat energy output from the heating device tends to change even though the heating device is operated with the same operation parameters. For instance, the heating temperature of the heating device gradually rises as the TLD reader processes the badges because of heat accumulated in the heating device. Also, the heating temperature of the heating device changes as the heating device ages. If the heating temperature exceeds or falls short of the optimum level, reading of radiation doses by the TLD reader will no longer be accurate.
Numerous attempts have been made to provide the TLD reader with a heat sensor for measuring in real-time the temperature of a TL element being heated. The TLD reader, if provided with such a heat sensor, could display to an operator the temperature of a TL element being heated or could alert the operator to deviation of the heating temperature from the optimum temperature level. But these attempts have all been unsuccessful. The problem is that a heat sensor cannot be positioned in place near the TLD element being heated. The heating device must be placed on one side of the TLD plate as closely to the target TL element as possible in order to excluding any outside thermal disturbances and heat all the TL elements uniformly. The photomultiplier must be placed on the other side of the TLD plate as closely to the target element as possible because thermoluminescence from the element is so weak. Simply, there is no physical space for any heat sensor near the TLD element being heated.